The conventional optical communication system includes an optical transmitter provided with a semiconductor laser. The semiconductor laser is intensity-modulated by changing injection current corresponding to an information signal, so that the semiconductor emits a modulated laser light including the information signal. Such a modulation system is called as a direct modulation system. The modulated laser light emitted from the semiconductor laser is transmitted through an optical fiber and reaches an optical receiver. The optical receiver detects the modulated laser light and discriminates the information signal therefrom. In this conventional optical communication system, there occurs a frequency chirping in the modulated laser light, so that the information signal discriminated from the modulated laser light may deteriorate on certain conditions.
In the state of the art of optical communication system, a laser light is modulated in intensity by an external modulator in accordance with an information signal after being emitted from a semiconductor laser. The external modulator consists of a compound semiconductor such as InGaAsP or LiNbO.sub.3.
Such a modulation system is called as an external modulation system. Typical example of this optical communication system has been disclosed on pages 1357 to 1362 of "IEEE Journal of Lightwave Technology, vol. 8, 1990". Further, an optical fiber amplifier provided with an Er-doped optical fiber as an amplifying medium has been disclosed on pages 1175 to 1192 of "Japanese Journal of Applied Physics, vol. 59, 1990". Consequently, it has been realized to obtain an optical communication system having an output power over 20 dBm with little frequency chirping.
According to the conventional optical transmitters described above, however, there is a disadvantage in that the input signal light power is limited because of occurrence of the stimulated Brillouin scattering. The reason why the stimulated Brillouin scattering occurs has been described on page 2489 of "Applied Optics, vol. 11, 1972" and on page 710 of "IEEE Journal of Lightwave Technology, vol. 6, 1988". The stimulated Brillouin scattering is induced by a high intensity energy of the signal light to be transmitted in a single-mode optical fiber which has a core of a small diameter provided in the optical communication system. The stimulated Brillouin scattering tends to occur when an input signal light having little frequency chirping is coupled in an optical fiber, because the Brillouin gain bandwidth of the optical fiber is less than 100 MHz. If the stimulated Brillouin scattering occurs, most of the signal light is converted to a stimulated Brillouin scattering light and scattered backward toward the input facet of the optical fiber, so that the transmission signal power is reduced. In other words, even if the input signal power is increased, the signal power which reaches the optical receiver can not increase, so that the effective transmission length is limited. Further, the power level change of the signal light detected by the optical receiver occurs if the stimulated Brillouin scattering occurs. Therefore, it is required to avoid the stimulated Brillouin scattering to realize a long distance optical communication by increasing the transmission power level.